You may be tempted to use the iconic equation for the electrostatic potential to give an answer along the lines of $$V=k\left(\frac{q_1}{r_1}+\frac{q_2}{r_2}\right)$$ But this expression doesn't provide enough information to describe $r_1$ and $r_2$, a good example of the need to “unpack” the iconic equation.

Your final answers should explicitly involve $x,y,z$ and $D$.

In some cases, the contributions from the two terms cancel. How does this cancellation relate to the symmetries of the problem?